1. Field of Invention
The invention covers a three point gauge for measurement of external thread forms and OD's of parts. The gauge is both adjustable and has wear evident contact points. The gauge which has direct contact between an interior cam surface and part contacting rollers is especially effective in determination of thread classes by use of annular threaded rollers adjusted for various thread pitches. This gauge can be hand set or attached to indicating devices.
2. Prior Art
Metrology, the science of measurement, requires immense amounts of size indicators and standards to check each size indicator. In the case of measurement of outside diameter of objects, especially round outside diameters (OD's) of shafts, bolts and other objects, the size is often measured by the supplier, and the size is checked on receipt or in production by use of a series of go or no go gauges. In the case of threaded parts, bolts, shafts, or fasteners there are the further complications of several thread fit standards and various pitches of threads. The covering of all fits and pitches of a given size would require 12 gauges and there still might be off standard sizes not covered. In most cases, measurement is by a variable gauge with an indicator of size, or for more ease of use, by go and no-go gauges which are fixed rings or slots. There is a need for a go and a no-go gauge for each size and class being tested. In critical applications the step between go and no go may be of the magnitude of 0.0001 inch or less, thus a large number of gauges of very high accuracy are needed and they must be constantly checked to insure there is no wear which would affect the accuracy of the gauge. This is critical in many high technology applications where bolts or pins are used to both fasten and to align parts. The resultant need for accuracy in sizing has provided a major industry in standard and special gauges for pins, bolts, and special parts. The machine shop or precision assembly shop may have to store a vast number of gauges. They must have the go and no-go gauges, standards, and backups for replacement. Gauges thus can represent a major capital investment for these shops. At present gauges are for one fixed size only. There is a need for gauges that provide accurate go and no-go measurements but which can be easily set to cover a variety of sizes and/or classes of fit.
The basic metrology problem is made much worse by a common ploy of manufacturers to make parts slightly different that the standard sizes to thus require the use of the manufacturers replacement parts rather than shelf replacement parts. As a result there is a vast number of parts that are close to standard size but differ by just enough to require separate gauges and test equipment. The number of sizes further emphasizes the unmet need for a simple but variable setting go no-go gauge system.
Gauges are often precision ground holes in a hardened metal part. Such gauges are easy to manufacture but they are hard to use and can be easily damaged. The typical gauge is thus a round hole gauge with a sliding fit over part sizes.
Machinists have long used vee blocks and holddowns to hold and fix round or irregular shaped OD parts prior to machining. The vee block is usually either a 90 or a 120 degree included angle hardened steel surface. For the most exacting uses it may be made from granite to curb wear. The 120 degree angled blocks are especially favored. The use of the vee blocks was recognized by gauge manufacturers and the concept is incorporated in many gauges.
The initial simple hole gauge concepts are common in go and no go gauges. In measuring, where there is an actual indicator of size, these hole gauges have largely been abandoned and three point gauges are common. The three point system uses the concepts of the vee block holddowns. The prevalent type of gauge is a vee shaped anvil where the enclosed angle of the anvil vee is 120 degrees and where the anvil thus holds a round part at two contact points with a micrometer indicator reading a third point offset by 120 degrees from the two anvil contact points. Such a system is accurate, and by mere modification of the micrometer scale is direct reading. Since reading a micrometer is a skill and since a micrometer can easily be misread, go and no-go gauges are still the main QC check on parts.
The vee anvil micrometer and similar gauges have been improved with free spinning rollers, offset by 120 degrees from each other, providing the points of contact with parts that were done with the 120 degree angled anvil. The use of the limited contact of this three point contact system provides less surface in contact with the parts without loss of accuracy. In fact, since dents are eliminated in the gauging surfaces, the accuracy is often much better with the three point system. The accuracy is also enhanced by the fact that the three point roller gauges have the rollers freely rotating so that the surface in contact with the parts is randomized and wear is well distributed over the roller surfaces. The downside to this roller system is that wear accumulates in the roller system and the system slowly starts to give non-standard measurements. Usually this gradual side in accuracy is very hard to notice and may give marginal readings on parts for a period of time before it is clearly identifiable. There is a need for a simple and effective method to use the advantages of the three point system and still be able to detect wear and cure this wear. There is also a need to provide the easy of use of a direct reading system and its adjustability to a go and no-go gauge system. A further advantage of such a system would be a greatly reduced gauge inventory.
Expansions of the anvil and the multipoint or three point gauge have been made. In many of these gauges, complex lever arm and/or spring loaded arms are used that add linkages or cumulative tolerances that result in gauge variations. Key to an effective gauge is a minimum number of parts in linkages and a direct metal to metal contact between all parts that eliminates cumulative error and linkage slide, thus making the gauge more accurate. Three point systems usually have linkage or pressure sensitive elements that add error. An ideal gauge would eliminate these linkage based sources of error.
There are further problems in a specific area of metrology, the measurement of thread fit. A series of thread classes are used to indicate the thread fit characteristics. To measure a given class of fit, a special contact thread is needed for the go and no-go gauge that is totally traceable to QC standards. If two different fit classes are checked in a size of bolt, thus 4 gauges would be needed in addition to test standards. Each fit class requires a different thread also. This is further compounded by the prevalence of special sizes that are only close to the standard sizes. Such specials are common to force users to buy parts from the manufacturers. Each special also needs a set of gauges to test it. As can be noted, the number of gauges rapidly becomes a major expense. There is a need for a test procedure that minimizes the need for gauges.